Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TITLE: Gas-phase process and apparatus for the polymerization of olefins
The present invention relates to a process and apparatus for the gas-phase
polymerization
of a-olefins carried out in the presence of a polymerization catalyst system.
In particular,
the invention relates to gas-phase polymerization in a continuously operated
fluidized bed
reactor provided with equipment for the continuous discharge of polymer
powder.
The development of catalysts with high activity and selectivity of the Ziegler-
Natta type and,
more recently, of the metallocene type has led to the widespread use on an
industrial scale of
processes in which the olefin polymerization is carried out in a gaseous
medium in the
presence of a solid catalyst. An example of said gas-phase polymerization
processes involves
the use of a fluidized bed reactor wherein a bed of polymer particles is
maintained in a
fluidized state by the upward flow of a fluidizing gas.
During the polymerization fresh polymer is generated by catalytic
polymerization of the
monomers and the manufactured polymer is drawn off from the reactor to
maintain the
polymer bed at a constant volume. The fluidized bed, which comprises a bed of
growing
polymer particles and catalyst particles, is maintained in a fluidization
state by the
continuous upward flow of a fluidizing gas, which comprises the recycled gas
stream and
make-up monomers. Industrial processes employ a distribution plate to dispense
the
fluidizing gas to the polymer bed, the distribution plate acting also as a
support for the bed
when the supply of gas is cut off. The fluidizing gas enters the bottom of the
reactor and is
passed through the distribution plate to the fluidized polymer bed.
The polymerization of olefins is an exothermic reaction and it is therefore
necessary to
provide means to cool the bed to remove the heat of polymerization. In the
absence of such
cooling the bed would increase in temperature until, for example, the catalyst
turns inactive
or the polymer particles are partially fused. When polymerizing in a fluidized
bed reactor,
the preferred method for removing the heat of polymerization is by feeding to
the reactor a
recycle gas stream at a temperature lower than the desired polymerization
temperature.
Such a recycle stream, while passing through the fluidized bed, allows
conducting away
the heat of polymerization. The recycle gas stream is withdrawn from the upper
zone of the
fluidized bed reactor, cooled by passage through an external heat exchanger
and then
recycled to the reactor. The temperature of the recycle gas stream can be
adjusted in the
heat exchanger to maintain the fluidized bed at the desired polymerization
temperature.
The recycle gas stream generally comprises, besides the gaseous monomers, also
inert and
diluent gases, such as propane or higher saturated hydrocarbons and/or
nitrogen, and
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eventually gaseous chain transfer agents, such as hydrogen. Monomers and chain
transfer
agents, consumed by the polymerization reaction, are normally replaced by
adding make-
up gas to the recycle gas stream.
It is known that the discharge of polymer powder from a fluidized bed reactor
may be
carried out discontinuously or continuously. Bearing in mind the high
pressures adopted in
the gas-phase polymerization, the conventional discontinuous systems of
unloading solids
generally comprise at least one intermediate reservoir connected upstream to
the reactor
and downstream to a holding reservoir by means of pipes, each of which is
equipped with a
valve. These valves operate in sequence, so that the unloading system operates
like a lock.
A fraction of the formed polymer together with the reaction gas are first
discharged into
the intermediate reservoir, and then into the holding reservoir. Correct
operation of the
intermediate reservoir generally requires the use of a pressure-equalizing
system placing
the intermediate reservoir in communication with the top of the reactor. An
unloading
system of this type is described in GB patent 1375741.
Various versions or modifications of the lock principle are described or
mentioned in some
European patents. For instance, EP-88655 and EP-202076 disclose a high speed
evacuation
system of the fluidized bed, for example when changing the type of polymer
produced in
the reactor, which comprises a vertical evacuation pipe, which is centrally
connected to a
fluidization grid, said vertical evacuation pipe being provided with a high
speed on/off
valve. These discharge systems are described as an auxiliary system
complementary to the
traditional discharge systems based on a lateral pipe placed above the
fluidization grid.
European Patent Application EP 250169 describes a system comprising two
rotating plug
valves, which are driven in such a way that they are never both open
simultaneously. A
depressurization tank is connected to the fluidized bed reactor and is
interposed between
said two rotating plug valves in order to reduce the pressure fluctuations
inside the
fluidized bed reactor, arising as a result of the discontinuous discharge of
polymer powder
from the reactor.
All these unloading systems operate discontinuously and require the use of
valves which
open in sequence. This discontinuous method of operation is often the cause of
fluctuations
in pressure and/or flow rate, or fluctuations in the level of the fluidized
bed inside the
reactor, when a batch of manufactured polymer is withdrawn from the reactor.
These
fluctuations influence the monomer concentration and also other parameters,
such as the
concentration of chain transfer agents and co-monomers, which all taken
together have a
strong impact on the quality of the polymer product. Furthermore, the
disruptions caused
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by the above fluctuations have a negative effect on the operation of the
equipments placed
downstream the polymerization reactor, causing for instance wide variations of
pressure at
the outlet of the discharge valve, thus requiring great volumes downstream the
discharge
valve in order to reduce any pressure oscillation. The discontinuous discharge
system is
also rather expensive as an investment, and also burdensome in maintenance
costs. These
drawbacks make the above discontinuous systems not the best option to be used
in modem
industrial processes for the olefin polymerization.
A fluidized bed reactor can better work in a steady and reliable way only if
the discharge
of polymer powder is operated and adjusted in a continuous way. In fact, even
minimal
fluctuations on the operating conditions (temperature, pressure, monomers
concentration)
can considerably increase or decrease the production rate of polymer.
The currently adopted solution is represented by a continuous discharge of
polymer
through a discharge valve placed in the bottom region of the fluidized bed of
polymer. In
particular, the manufactured polymer in a powder form is generally drawn off
from the
reactor by at least one side discharge conduit situated along the vertical
wall of the reactor
above the fluidization grid, and is then subjected to a decompression and
degassing stage.
Segmental ball valves or eccentric rotary type valves are commonly used as
control valves
at the outlet from a fluidized bed polymerization reactor. This type of
discharge system
gives the advantage of not creating stagnant zones and consequent local hot
spots in the
region of the fluidized bed nearer to the outlet from the reactor. By
maintaining a
sufficiently low pressure downstream the discharge valve, the polymerization
reaction
practically ceases due to the low partial pressure of the monomer, thus
avoiding risk of
polymerization in the receiving apparatus downstream.
According to the disclosure of EP 1159305 free-flowing polymer powder is
continuously
withdrawn from a fluidized bed reactor via a discharge pipe, while
simultaneously
monitoring the surface level of the fluidized bed within the gas-phase
reactor. The flow of
polymeric material through the discharge pipe is controlled, so as to maintain
an essentially
constant bed level into the reactor. To achieve this aim, the reactor is
provided with an
outlet nozzle equipped with a continuously operated control valve for the
polymer
discharge. The discharge system of EP 1159305 comprises an outlet nozzle from
the
fluidized polymer bed, a collecting vessel in communication with said outlet
nozzle for
separating gas from the solid material, a control valve and a bed level
indicator. Ball
valves, V-ball valves and hose valves are mentioned as the continuously
operated control
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valves. Both the discharge line and the control valve are discontinuously back-
flushed with
a flushing gas flow to prevent their clogging.
However, the great disadvantage associated with this type of continuous
discharge system
is correlated to the excessive amount of gaseous monomers and inert compounds
which are
continuously discharged from the fluidized bed together with the polymer
powder. In fact,
the amount of reaction mixture accompanying the polymer is high, being the
polymer
drawn off from a reaction zone where the solids concentration is quite low.
Said
considerable amount of reaction monomers cannot be wasted and has to be
necessarily
recovered by the use of large-sized devices for decompressing and degassing
the polymer,
and also appropriate and costly devices for recompressing and recycling part
of said
reaction gas mixture into the polymerization reactor. Higher is the amount of
reaction gas
mixture to be recovered and recycled to the reactor, higher are the operative
costs which
are involved.
It is therefore largely felt the need of overcoming the mentioned
disadvantages associated
with the conventional prior art discharge systems, providing an innovative
process for
continuously discharging polymer powder from a gas-phase polymerization
reactor that
can simultaneously reduce the amount of associated gas.
It has now been found a process and apparatus for the gas-phase polymerization
of olefins
capable of minimizing in a considerable way the amount of gas withdrawn from
the
polymerization apparatus together with the polymer.
It is therefore a first object of the present invention a gas-phase process
for polymerizing
one or more a-olefins in a fluidized bed reactor in the presence of a
polymerization
catalyst, said fluidized bed reactor being equipped with a fluidization grid
arranged at its
base, and external means for recycling and cooling the unreacted gas from the
top of said
reactor to said fluidization grid, the process being characterized by:
(i) a continuous pneumatic recycle of polymer by means of a circulation loop
connecting said fluidization grid to the upper region of the fluidized bed
reactor;
(ii) a continuous discharge of polymer from a zone of said circulation loop
having a
polymer concentration higher than the polymer concentration inside the
fluidized
polymer bed.
The gas-phase polymerization process of the present invention comprises a
continuous
pneumatic recycle of polymer particles from a region situated in the bottom
portion of
the fluidized bed to a region situated in the upper part of the fluidized bed
reactor. The
technical feature (i) of the present invention is carried out by means of a
circulation loop
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running outside the fluidized bed reactor, said circulation loop comprising a
substantially
vertical standpipe and a pneumatic conveyor pipe.
The substantially vertical standpipe protrudes downwardly from the
distribution grid so
that polymer powder coming from the fluidized bed falls due to gravity into
said standpipe.
A solid thickening zone is formed in this standpipe since the polymer is
forced to flow, by
gravity, inside a restricted volume: this allows the implementation of the
technical feature
(ii) of the present invention because said solid thickening zone is
characterized by a
concentration of solid higher than the solids concentration present inside the
fluidized bed
reactor.
The polymer powder flows by gravity in said solid thickening zone and this
continuous
flow prevents from the formation of "hot spots" in the standpipe of the
circulation loop. A
discharge valve can be advantageously placed in correspondence of said
thickening zone
for continuously discharging a fraction of polymer powder flowing inside the
circulation
loop. As a consequence, the amount of gas discharged with the polymer is
drastically
reduced in comparison with the prior art technique of discharging polymer
directly from
the fluidized bed of polymer: in fact, in correspondence of said thickening
zone the
quantity of gas is brought to values close to those of the intergranular gas
surrounding the
polymer particles in a packed bed condition.
The implementation of the technical feature (ii) of the invention leads to
drastic reduction
in volumes of unreacted gases to be recovered and a consequent remarkable
reduction of
operative costs downstream the discharge valve, specifically in the stages of
decompressing and degassing the polymer, and in recompressing and recycling
the reaction
gas mixture to the polymerization apparatus.
As said, the polymer discharge valve is advantageously placed in
correspondence of said
solid thickening zone and the opening of said valve is continuously adjusted
so as to keep
constant the height of the fluidized polymer bed inside the reactor. According
to the
invention, the ratio between the flow rate of polymer continuously recycled to
the reactor
via the circulation loop and the flow rate of polymer continuously discharged
through said
discharge valve is comprised between 2 and 20, preferably between 4 and 15.
Downstream the vertical standpipe, the circulation loop of the invention
comprises a
pneumatic conveyor pipe having the function of reintroducing into the
fluidized bed
reactor the polymer powder which by-passes the discharge valve: a "thrust" gas
is fed at
the inlet of said pneumatic conveyor pipe to enable the continuous pneumatic
recycle of
polymer to the upper region of said fluidized bed reactor. This thrust gas is
generally taken
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from the gas recycle system for recycling the unreacted monomers from the top
of the
fluidized bed reactor to below the fluidization grid.
At the end of the pneumatic conveyor pipe the polymer powder is reintroduced
into the
fluidized bed reactor, preferably in a region above the polymer bed and below
the velocity
reduction zone, if present in the fluidized bed reactor.
In conventional fluidized bed reactors the broad particle size distribution
may cause
elutriation and segregation effects with bigger particles concentrating more
in the lower
region of the polymer bed, so that a different particle size distribution can
be found at
different level of the fluidized bed.
The continuous circulation of solid from the bottom to the upper region of the
fluidized
bed reactor gives the further advantage of improving the uniformity of the
polymer bed.
A best homogeneity in the particle size distribution of the polymer bed is
achieved because
the solid fraction having a higher average diameter, which would tend to
segregate from
the bed and accumulate on the gas distribution grid, is advantageously
conveyed
downwards towards the standpipe, and from there discharged outside the reactor
or
recycled to the top of the fluidized bed.
Other advantages and features of the present invention are illustrated in the
following
detailed description with reference to the attached drawing, which is
representative and not
limitative of the scope of the invention.
Figure 1 is a fluidized bed reactor having a polymer discharge system
according to the
process of the present invention.
With reference to Figure 1, a fluidized bed reactor 1 for the continuous
polymerization in
gas-phase of a-olefins is shown. The reactor 1 comprises a fluidized bed 2 of
polymer, a
fluidization grid 3 and a velocity reduction zone 4. The velocity reduction
zone 4 is
generally of increased diameter compared to the diameter of the fluidized bed
portion of
the reactor. The polymer bed is kept in a fluidization state by an upwardly
flow of gas fed
through the fluidization grid 3 placed at the bottom portion of the reactor 1.
The gaseous stream leaving the top of the velocity reduction zone 4 comprises,
besides the
unreacted monomers, also inert condensable gases, such as alkanes, as well as
inert non-
condensable gases, such as nitrogen. Said gaseous stream is compressed, cooled
and
recycled to the bottom of the fluidized bed reactor: from the top of the
velocity reduction
zone 4 the gaseous stream is transferred via recycle line 5 to a compressor 6
and then to a
heat exchanger 7. Passing through the heat exchanger 7, the gaseous stream is
cooled in
order to dissipate the reaction heat and then transferred to the bottom of the
fluidized bed
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reactor (below the gas distribution grid) via line 8. Make-up monomers,
molecular weight
regulators, and optional inert gases are fed into the reactor 1 via line M. In
figure 1 the
position of line M is placed, just as an example, upstream the compressor 6,
this non
limiting the scope of the invention.
Generally, the various catalyst components are fed to the reactor 1 via a line
9 that is
preferably placed in the lower part of the fluidized bed 2.
The fluidized bed reactor 1 of the invention is provided with a continuous
pneumatic
recycle of polymer by means of a circulation loop, indicated with reference R,
connecting
said fluidization grid 3 to the upper region of the fluidized bed reactor 1.
The circulation loop R comprises a substantially vertical standpipe 10, that
may be made of
a uniform diameter, or preferably comprises more sections having decreasing
diameters in
the downward direction. The inlet of the standpipe 10 is connected to the
fluidization grid
3 while its lower part is connected to a pneumatic conveyor pipe 11, which has
the
function of reintroducing the polymer powder into the fluidized bed reactor 1.
The outlet of
said pneumatic conveyor pipe 11 is preferably placed above the polymer bed 2
and below
the velocity reduction zone 4.
The gas distribution grid 3 may be flat, but preferably is endowed with a cone
shape in
such a way that its downward inclination towards the standpipe 10 fosters the
entry of the
polymer powder into the standpipe 10 due to gravity. Polymer powder enters the
standpipe
without the addition of any gas, and gives rise to the formation of a solid
thickening
zone with a positive pressure gradient. The inlet of the standpipe 10 is
preferably located in
a central position with respect to the fluidization grid 3, as shown in Fig.
1.
In the standpipe 10 the polymer flows downwards under the action of gravity so
that the
density of the solid (kg/m) therein is higher than the density in the
fluidized bed 2, said
density in the standpipe 10 being close to the bulk density of the polymer.
A control valve 12 is installed in proximity of standpipe 10 for adjusting the
flow rate of
polymer discharged from the reactor 1 into the discharge conduit 13.
Preferably, when the
standpipe 10 is formed with decreasing diameters, the control valve 12 is
placed in
correspondence of a restriction existing between a section of higher diameter
l0a and a
section of lower diameter l0b as shown in Fig. 1. An on-off safety valve l0c
is placed on
said restriction, the closure of said valve l0c causing the interruption of
the solid
recirculation along the circulation loop R.
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Segmental ball valves or eccentric rotary type valves can be used as the
control valve 12.
The discharge of polymer is carried out in continuous and the opening of said
control valve
12 is adjusted so as to keep constant the level of solid inside the fluidized
bed reactor 1.
The solid not discharged through the discharge conduit 13 is recycled to the
upper region
of the fluidized bed reactor 1 by the circulation loop R.
A "thrust gas" is generally fed via line 14 at the inlet of the pneumatic
conveyor pipe 11,
said thrust gas being the carrier entraining the solid particles along the
conveyor pipe 11.
The regulation of the mass flow rate of solid recycled through the circulation
loop R can be
carried out by means of the control valves 14b and 15, adjusting the flow rate
of "thrust
gas" entering the conveyor pipe 11. Said thrust gas can advantageously be
taken from the
gas recycle line at a point downstream the compressor 6 and upstream the heat
exchanger
7, thus exploiting the pressure drop existing through the heat exchanger 7,
the distribution
grid 3 and the polymer bed 2.
The operative pressure in the reactor 1 is maintained at conventional values
generally
comprised between 10 and 30 bar, the temperature being comprised between 40
and
130 C.
The pressure downstream the control valve 12 is preferably in the range
between 0.5 and 3
bar, sufficiently low to stop the polymerization reaction and cause the
devolatilization of
most of the gas dissolved in the solid. Obviously, the operative conditions
downstream the
discharge valve 12 cause an instantaneous evaporation (flash) of hydrocarbons
dissolved in
the polymer powder. The polymer discharged through the control valve 12 is
transported
via the discharge conduit 13 up to a separation tank 16.
The gas accompanying the polymer discharged through the discharge valve 12 and
also the
gas released by depressurization are separated from the polymer in the
separation tank 16.
The separated gas may be recycled to reactor 1 by means of the compressor 19
via line 17.
Another introduction point, i.e. line 18, can be placed along line 17 to
supply monomers,
chain regulators and optionally inert gas.
From the bottom of the separation tank 16 the polymer is recovered and may be
conveyed
via line 21 to the successive steps of stripping hydrocarbons dissolved in the
polymer,
deactivation of catalyst residues or, in alternative, to a successive
polymerization step.
Another advantage achieved from the process of the invention and correlated to
the
continuous pneumatic recycle of polymer powder to the reactor regards the
possibility of
feeding liquid monomers and/or catalyst components (for example metal-alkyls)
directly
into the circulation loop R. This may be carried out, for instance, by means
of line 20,
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advantageously placed in correspondence of the upper portion of the conveyor
pipe 11. In
the case of feeding a liquid monomer, an effective dispersion of the latter on
the solid
particles and its immediate vaporization is obtained, with the consequent
advantage of
improving the removal of polymerization heat from the polymerization reactor.
Furthermore, with respect to the conventional feeding of make-up monomers at
traditional
points along the gas recycle line 17, the fouling problems caused by the
presence of
electrostatic charges do not occur if make-up monomers and/or reaction
activators are fed
via line 20 to the conveyor pipe 11. This is due to the fact that inside the
circulation loop R
the electrostatic forces have virtually no influence due to the high momentum
of the solid
and its cleaning effect on the walls of the loop R.
According to a second embodiment of the invention, the olefin polymerization
in the above
described process and apparatus can be operated also in the so called
"condensing mode".
This technique is generally exploited for increasing the space time yield in a
continuous
fluidized bed reactor. The recycle gas stream is intentionally cooled to a
temperature below
its dew point to produce a two-phase mixture under conditions such that the
liquid phase of
said mixture will remain entrained in the gas phase of said mixture. Said two-
phase
mixture is introduced into the reactor at a point in the lower region of the
reactor, and most
preferably at the bottom of the reactor to ensure uniformity of the fluid
stream passing
upwardly through the fluidized bed. The evaporation of the liquid phase takes
place inside
the polymerization bed and this ensures a more effective removal of the
polymerization
heat.
By operating in the "condensing mode", the cooling capacity of the recycle
stream is
increased by the vaporization of the entrained condensed liquids and also in
view of the
higher temperature gradient existing between the recycle stream and the
reactor.
It is therefore a second object of the present invention a gas-phase process
for
polymerizing one or more a-olefins in a fluidized bed reactor in the presence
of a
polymerization catalyst, said fluidized bed reactor being equipped with a
fluidization grid
arranged at its base, and external means for recycling and cooling the
unreacted gas from
the top of said reactor to said fluidization grid, the process being
characterized by:
(i) a continuous pneumatic recycle of polymer by means of a circulation loop
connecting said fluidization grid to the upper region of the fluidized bed
reactor;
(ii) a continuous discharge of polymer from a zone of said circulation loop
having a
polymer concentration higher than the polymer concentration inside the
fluidized
polymer bed;
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(iii) the introduction of a two-phase mixture of gas and liquid under the
fluidization
grid, said two-phase mixture coming from said external means for recycling and
cooling the unreacted gas.
With reference to Fig. 1, the two-phase mixture introduced according to
feature (iii) under
the fluidization grid 3 of the fluidized bed 2 comprises one or more monomers
of formula
CHz=CHR, where R is hydrogen or a hydrocarbon radical having 1-12 carbon atoms
and
optionally also one or more Cz-Cg alkanes or cycloalkanes as inert condensable
gases.
The condensed liquid moves upwardly through the distribution grid 3, so that
its
evaporation contributes to provide an improved cooling of the lower region of
the fluidized
bed 2 and thus of the polymer entering the circulating pipe R of the present
invention. This
has the beneficial effect of providing a partial cooling of the polymer powder
while
circulating inside the circulation loop.
All the above described advantages of the present invention can be achieved by
using a
gas-phase polymerization apparatus, here described in connection with Fig. 1.
It is therefore a further object of the present invention an apparatus for the
gas-phase
polymerization of olefins in a fluidized bed reactor, said fluidized bed
reactor being
equipped with a fluidization grid arranged at its base, a gas circulation
system, and a
device for the continuous discharge of polymer from the reactor, characterized
by the fact
that said discharge device comprises:
- a pneumatic circulation pipe (R) comprising a substantially vertical
standpipe 10
and a pneumatic conveyor pipe 11, said standpipe 10 defining a solid
thickening
zone in which the concentration of solid is higher than the solid
concentration in the
polymer bed 2,
- a solid discharge conduit 13 connected to said standpipe 10 by means of a
regulation means 12 suitable for adjusting the mass flow rate of polymer
discharged
from the reactor 1.
The circulation loop R connects the fluidization grid 3 to the upper region of
the fluidized
bed reactor, preferably to a region above the polymer bed 2 and below the
velocity
reduction zone 4. The gas distribution grid 3 has preferably a conical shape
surrounding
the standpipe 10, the inlet of said standpipe 10 being placed at the center of
said
distribution grid 3.
As regards the average diameter of the circulation loop R, this parameter is
generally
selected at a value of less than 0.15 DR, where DR is the diameter of the
fluidized bed
reactor. Above this upper limit, an excessive amount of gas is required to
circulate into the
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loop R, so that a compressor 6 of increased size has to be adopted in the gas
recycle line. A
suitable range for the diameter of the circulation loop R is from 0.01 to 0.15
DR, preferably
from 0,02 to 0,08 DR.
The regulation means 12 for adjusting the amount of discharged polymer
comprises a
control valve, interposed between the standpipe 10 and the discharge conduit
13.
The standpipe 10 may be made of a uniform diameter, but preferably comprises
more
sections having decreasing diameters in the downward direction. The control
valve 12 is
preferably placed in correspondence of a restriction between a section of
higher diameter
l0a and a section of lower diameter l Ob as shown in Fig. 1.
The discharge conduit 13 connects said regulation means 12 to a separation
tank 16,
wherein the obtained polymer is separated from the gas, which is recycled to
the reactor
via line 17 and recompression means 19.
The polymerization apparatus of the present invention further comprises:
- one or more lines, such as line 14, for feeding a thrust gas at the inlet of
said
pneumatic conveyor pipe 11;
- regulation means, such as control valves 14b and 15, for adjusting the flow
rate of
said thrust gas;
- means 20 for feeding liquid monomers and/or catalyst components directly
into
said circulation loop R, said means being placed in correspondence of the
upper
portion of said conveyor pipe 11.
The polymerization process of the invention can be combined with conventional
technologies operated in slurry, in bulk, or in a gas-phase, to carry out a
sequential
multistage polymerization process. Therefore, upstream or downstream the
apparatus of
the invention, one or more polymerization stages operating in a loop reactor,
or in a
conventional fluidized bed reactor, or in a stirred bed reactor, can be
provided. In
particular, gas-phase polymerization reactors having interconnected
polymerization zones
as described in EP 782 587 and EP 1012195 can be advantageously operated
upstream or
downstream the apparatus of the present invention.
The gas-phase polymerization process of the invention allows the preparation
of a large
number of olefin powders having an optimal particle size distribution with a
low content of
fines. The a-olefins preferably polymerized by the process of the invention
have formula
CHz=CHR, where R is hydrogen or a hydrocarbon radical having 1-12 carbon
atoms.
Examples of polymers that can be obtained are:
- high-density polyethylenes (HDPEs having relative densities higher than
0.940)
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including ethylene homopolymers and ethylene copolymers with a-olefins having
3 to 12
carbon atoms;
- linear polyethylenes of low density (LLDPEs having relative densities lower
than
0.940) and of very low density and ultra low density (VLDPEs and ULDPEs having
relative densities lower than 0.920 down to 0.880) consisting of ethylene
copolymers with
one or more a-olefins having 3 to 12 carbon atoms;
- elastomeric terpolymers of ethylene and propylene with minor proportions of
diene
or elastomeric copolymers of ethylene and propylene with a content of units
derived from
ethylene of between about 30 and 70% by weight;
- isotactic polypropylene and crystalline copolymers of propylene and ethylene
and/or other a-olefins having a content of units derived from propylene of
more than 85%
by weight;
- isotactic copolymers of propylene and a-olefins, such as 1-butene, with an a-
olefin
content of up to 30% by weight;
- impact-resistant propylene polymers obtained by sequential polymerisation of
propylene and mixtures of propylene with ethylene containing up to 30% by
weight of
ethylene;
- atactic polypropylene and amorphous copolymers of propylene and ethylene
and/or
other a-olefins containing more than 70% by weight of units derived from
propylene.
The gas-phase polymerization process herewith described is not restricted to
the use of any
particular family of polymerization catalysts. The invention is useful in any
exothermic
polymerization reaction employing any catalyst, whether it is supported or
unsupported,
and regardless of whether it is in pre-polymerized form.
The polymerization reaction can be carried out in the presence of highly
active catalytic
systems, such as Ziegler-Natta catalysts, single site catalysts, chromium-
based catalysts,
vanadium-based catalysts.
A Ziegler-Natta catalyst system comprises the catalysts obtained by the
reaction of a
transition metal compound of groups 4 to 10 of the Periodic Table of Elements
(new notation)
with an organometallic compound of group 1, 2, or 13 of the Periodic Table of
element.
In particular, the transition metal compound can be selected among compounds
of Ti, V, Zr,
Cr, and Hf. Preferred compounds are those of formula Ti(OR)nXy_n in which n is
comprised
between 0 and y; y is the valence of titanium; X is halogen and R is a
hydrocarbon group
having 1-10 carbon atoms or a COR group. Among them, particularly preferred
are titanium
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compounds having at least one Ti-halogen bond such as titanium tetrahalides or
halogenalcoholates. Preferred specific titanium compounds are TiC13, TiC14,
Ti(OBu)4,
Ti(OBu)C13, Ti(OBu)zC1z, Ti(OBu)3C1.
Preferred organometallic compounds are the organo-Al compounds and in
particular Al-alkyl
compounds. The alkyl-Al compound is preferably chosen among the trialkyl
aluminum
compounds such as for example triethylaluminum, triisobutylaluminum, tri-n-
butylaluminum, tri-n-hexylaluminum, tri-n-octylaluminum. It is also possible
to use
alkylaluminum halides, alkylaluminum hydrides or alkylaluminum sesquichlorides
such as
A1Et2C1 and A12Et3C13 optionally in mixture with said trialkyl aluminum
compounds.
Particularly suitable high yield ZN catalysts are those wherein the titanium
compound is
supported on magnesium halide in active form which is preferably MgC1z in
active form.
Particularly for the preparation crystalline polymers of CH2CHR olefins, where
R is a Cl
Cl0 hydrocarbon group, internal electron donor compounds can be supported on
the MgC1z.
Typically, they can be selected among esters, ethers, amines, and ketones. In
particular, the
use of compounds belonging to 1,3-diethers, cyclic ethers, phthalates,
benzoates, acetates
and succinates is preferred.
When it is desired to obtain a highly isotactic crystalline polypropylene, it
is advisable to
use, besides the electron-donor present in the solid catalytic component, an
external electron-
donor (ED) added to the aluminium alkyl co-catalyst component or to the
polymerization
reactor. These external electron donors can be selected among alcohols,
glycols, esters,
ketones, amines, amides, nitriles, alkoxysilanes and ethers. The electron
donor compounds
(ED) can be used alone or in mixture with each other. Preferably the ED
compound is
selected among aliphatic ethers, esters and alkoxysilanes. Preferred ethers
are the C2-C20
aliphatic ethers and in particular the cyclic ethers preferably having 3-5
carbon atoms, such
as tetrahydrofurane (THF), dioxane.
Preferred esters are the alkyl esters of Cl-C20 aliphatic carboxylic acids and
in particular
Cl-C8 alkyl esters of aliphatic mono carboxylic acids such as ethylacetate,
methyl
formiate, ethylformiate, methylacetate, propylacetate, i-propylacetate, n-
butylacetate, i-
butylacetate.
The preferred alkoxysilanes are of formula Ra1Rb2Si(OR3),, where a and b are
integer from 0
to 2, c is an integer from 1 to 3 and the sum (a+b+c) is 4; Ri, W, and R3, are
alkyl, cycloalkyl
or aryl radicals with 1-18 carbon atoms. Particularly preferred are the
silicon compounds in
which a is 1, b is 1, c is 2, at least one of R' and W is selected from
branched alkyl, cycloalkyl
or aryl groups with 3-10 carbon atoms and R3 is a C1-Cio alkyl group, in
particular methyl.
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Other useful catalysts are the vanadium-based catalysts, which comprise the
reaction
product of a vanadium compound with an aluminum compound, optionally in the
presence
of a halogenated organic compound. Optionally the vanadium compound can be
supported
on an inorganic carrier, such as silica, alumina, magnesium chloride. Suitable
vanadium
compounds are VC14, VC13, VOC13, vanadium acetyl acetonate.
Other useful catalysts are those based on chromium compounds, such as chromium
oxide
on silica, also known as Phillips catalysts.
Other useful catalysts are single site catalysts, for instance metallocene-
based catalyst
systems which comprise:
at least a transition metal compound containing at least one n bond;
at least an alumoxane or a compound able to form an alkylmetallocene cation;
and
optionally an organo-aluminum compound.
A preferred class of metal compounds containing at least one n bond are
metallocene
compounds belonging to the following formula (I):
Cp(L)qAMXp (I)
wherein M is a transition metal belonging to group 4, 5 or to the lanthanide
or actinide groups
of the Periodic Table of the Elements; preferably M is zirconium, titanium or
hafnium;
the substituents X, equal to or different from each other, are monoanionic
sigma ligands
selected from the group consisting of hydrogen, halogen, R6, OR6, OCOR6, SR6,
NR6z and
PR62, wherein R6 is a hydrocarbon radical containing from 1 to 40 carbon
atoms;
preferably, the substituents X are selected from the group consisting of -Cl, -
Br, -Me, -Et, -n-
Bu, -sec-Bu, -Ph, -Bz, -CH2SiMe3, -OEt, -OPr, -OBu, -OBz and -NMe2;
p is an integer equal to the oxidation state of the metal M minus 2;
n is 0 or 1; when n is 0 the bridge L is not present;
L is a divalent hydrocarbon moiety containing from 1 to 40 carbon atoms,
optionally
containing up to 5 silicon atoms, bridging Cp and A, preferably L is a
divalent group (ZR72)n;
Z being C, Si, and the R7 groups, equal to or different from each other, being
hydrogen or a
hydrocarbon radical containing from 1 to 40 carbon atoms;
more preferably L is selected from Si(CH3)2, SiPh2, SiPhMe, SiMe(SiMe3), CH2,
(CH2)2,
(CH2)3 or C(CH3)2;
Cp is a substituted or unsubstituted cyclopentadienyl group, optionally
condensed to one or
more substituted or unsubstituted, saturated, unsaturated or aromatic rings;
A has the same meaning of Cp or it is a NR7, -0, S, moiety wherein R7 is a
hydrocarbon
radical containing from 1 to 40 carbon atoms;
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Alumoxanes used as component b) are considered to be linear, branched or
cyclic compounds
containing at least one group of the type:
U U
A1 O Al/
U U
wherein the substituents U, same or different, are defined above.
In particular, alumoxanes of the formula:
U U U
Al O (Al O)ni - Al
U U
can be used in the case of linear compounds, wherein n' is 0 or an integer of
from 1 to 40 and
where the U substituents, same or different, are hydrogen atoms, halogen
atoms, C1-Czo-alkyl,
C3-Czo-cyclalkyl, C6-Czo-aryl, C7-C20-alkylaryl or C7-C20-arylalkyl radicals,
optionally
containing silicon or germanium atoms, with the proviso that at least one U is
different from
halogen, and j ranges from 0 to 1, being also a non-integer number; or
alumoxanes of the
formula:
U
(1 Al-O)n2
can be used in the case of cyclic compounds, wherein n2 is an integer from 2
to 40 and the U
substituents are defined as above.
The catalyst may suitably be employed in the form of a pre-polymer powder
prepared
beforehand during a pre-polymerization stage with the aid of a catalyst as
described above.
The pre-polymerization may be carried out by any suitable process, for
example,
polymerization in a liquid hydrocarbon diluent or in the gas phase using a
batch process, a
semi-continuous process or a continuous process.
The following examples will further illustrate the present invention without
limiting its scope.
EXAMPLES
Reactor Setup
A fluidized bed reactor having the configuration shown in Fig. 1 was used to
carry out the
olefin polymerization according to the process of the invention.
The design parameters of the fluidized bed reactor are the following:
Diameter of the reactor = 2.4 m
Diameter of the circulation loop R = 0.2 m
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Conical shape of the fluidization grid with a bottom apex.
The circulation pipe R comprises a substantially vertical standpipe 10 and a
pneumatic
conveyor pipe 11, the inlet of said standpipe 10 is placed in correspondence
of the center
of the distribution grid 3. The vertical standpipe 10 is formed by two
sections decreasing in
diameter l0a and 10b, of 0.35 m and 0.2 m, respectively. The height of section
l0a is of
0.6 m, while the height of section l0b is of 2.0 m
The control valve 12 for discharging the polymer is placed in correspondence
of the
restriction between the sections l0a and l Ob as shown in Fig. 1.
Example 1
Preparation of the solid catalyst component
The polymerization was carried out in the presence of a Ziegler-Natta catalyst
comprising
a solid catalyst component prepared with the procedure described in EP 541760
on page 7,
lines 1-16.
Triethylaluminium (TEAL) as a cocatalyst, and methylcyclohexyldimethoxysilane
as an
electron donor, were contacted with the above solid catalyst component
according to the
teaching given in Example 1 of EP 541760, lines 25-29. The molar ratio TEAL/Ti
is of
100.
Polymerization conditions
The above catalyst, prepolymerized with propylene, was fed via line 9 to the
fluidized bed
reactor of Fig. 1, where ethylene was polymerized using Hz as molecular weight
regulator
and in the presence of propane as an inert diluent.
Make-up propane, ethylene and hydrogen were fed via line M to the gas recycle
line 17.
Polymerization conditions: T = 80 C, p =24 bar
The following gas composition is maintained inside the reactor:
Ethylene 50% mol
Hydrogen 15% mol
Propane 35% mol
About 600 m3/h of a gaseous mixture coming from the gas recycle line, taken
from a point
upstream the heat exchanger 7, were continuously introduced by means of line
14 into the
circulation loop R: this "thrust gas" ensures the continuous transport of
solid along the
circulation loop R up to the upper region of the fluidized bed reactor.
Moreover, about 15
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m3/h of "thrust gas" were continuously fed to the circulation loop R via the
control valve
15: the flow rate of recycled solid was 40 t/h.
Polyethylene powder was continuously discharged from the fluidized bed reactor
via the
control valve 12, always maintaining the control valve 23 on line 22
completely closed.
The opening of the control valve 12 was adjusted so as to keep constant the
level of solid
inside the fluidized bed reactor.
The separator tank 16 was kept at a pressure of 0.5 bar: the pressure
downstream the
discharge valve 12 was of about 2 bar, so as to stop the polymerization
reaction and also to
cause a partial devolatilization of the gas dissolved into the solid
particles.
About 5 t/h of polyethylene were continuously discharged via the control valve
12: the
weight ratio between the recycled polymer (loop R) and the discharged polymer
was 8.
The total amount of gas discharged with the solid, measured at the top of the
separator tank
16, was equal to 300 Nm3/h (0 C, 1 bar).
Running was followed for several days, with regular runs, allowing the
production of
polyethylene for commercial use. Polyethylene granules of spherical shape
having an
average diameter of 1.6 mm were obtained.
Example 2 (comparative)
Ethylene was polymerized in the presence of the same catalyst components of
example 1,
and maintaining the same operative conditions as described in example 1 inside
the reactor
(monomer concentration, temperature, pressure).
Differently from example 1, the polymer particles were continuously discharged
from the
reactor according to the prior art technique of using a discharge valve,
placed in the bottom
region of polymer fluidized bed. Therefore, during the polymerization run the
control valve
12 and the valve l0c were maintained completely closed, so as to stop the
continuous
recycle of solid in the loop R and also the polymer discharge from the loop R.
The control valve 23 was kept open, so as to discharge polymer via line 22
into the
separation tank 16. The opening of control valve 23 is adjusted so as to keep
constant the
level of solid inside the fluidized bed reactor.
The separator tank 16 was kept at a pressure of 0.5 bar: the pressure
downstream the
discharge valve 23 was of about 2 bar, so as to stop the polymerization
reaction and also to
cause a partial devolatilization of the gas dissolved into the solid
particles.
Polyethylene granules of spherical shape having an average diameter of about
1.6 mm
were obtained.
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About 4.8 t/h of polyethylene were continuously discharged via line 22 into
the separation
tank 16. The total amount of gas discharged with the solid, measured at the
top of the
separator tank 16, was equal to 387 Nm3/h.
The comparison with Example 1 demonstrates that the process and polymerization
apparatus of the invention is able to lead to a remarkable reduction in
volumes of unreacted
gases (monomer and inert components) to be recovered and recycled to the
reactor. This
implies a remarkable reduction of operative costs in the stages of
decompressing and
degassing the polymer, recompressing and recycling the reaction gas mixture to
the
polymerization apparatus.
Example 3
Preparation of the solid catalyst component
The polymerization was carried out in the presence of a Ziegler-Natta catalyst
system
comprising:
- a titanium solid catalyst component prepared with the procedure described in
EP
395 083, Example 3, according to which diisobutyl phthalate is used the
internal
donor compound.
- triethylaluminium (TEAL) as the cocatalyst;
- dicyclopentyldimethoxysilane (DCPMS) as the external donor.
The above components were pre-contacted at a temperature of 25 C for 10
minutes in a
pre-contacting vessel, the weight ratio TEAL/sol.cat being of 5, the weight
ratio
TEAL/DCPMS being of 5.
Polymerization conditions
The above catalyst, prepolymerized with propylene, was fed via line 9 to the
fluidized bed
reactor of Fig. 1, where propylene was polymerized using H2 as molecular
weight regulator
and in the presence of propane as an inert diluent.
Make-up propane, propylene and hydrogen were fed via line M to the gas recycle
line 17.
Polymerization conditions: T = 80 C, p = 20 bar
The following gas composition is maintained inside the reactor:
Propylene 54.8% mol
Propane 45.0% mol
Hydrogen 0.2% mol
About 750 m3/h of a gaseous mixture coming from the gas recycle line, taken
from a point
upstream the heat exchanger 7, were continuously introduced by means of line
14 into the
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circulation loop R: this "thrust gas" ensures the continuous transport of
solid along the
circulation loop R up to the upper region of the fluidized bed reactor.
Moreover, about 20
m3/h of "thrust gas" were continuously fed to the circulation loop R via the
control valve
15: the flow rate of recycled solid was 70 t/h.
Polypropylene powder was continuously discharged from the fluidized bed
reactor via the
control valve 12, always maintaining the control valve 23 on line 22
completely closed.
The opening of the control valve 12 was adjusted so as to keep constant the
level of solid
inside the fluidized bed reactor.
The separator tank 16 was kept at a pressure of 0.5 bar: the pressure
downstream the
discharge valve 12 was of about 2 bar, so as to stop the polymerization
reaction and also to
cause a partial devolatilization of the gas dissolved into the solid
particles.
About 6 t/h of polypropylene were continuously discharged via the control
valve 12 with a
weight ratio between the recycled polymer (loop R) and the discharged polymer
of 11.7.
The total amount of gas discharged with the solid, measured at the top of the
separator tank
16, was of 235 Nm3/h.
Running was followed for several days, with regular runs, allowing the
production of
polypropylene for commercial use. Polypropylene granules of spherical shape
having an
average diameter of 2.0 mm were obtained.
Example 4 (comparative)
Propylene was polymerized in the presence of the same catalyst components of
example 3,
and maintaining the same operative conditions as described in example 3 inside
the
fluidized bed reactor (monomer concentration, temperature, pressure).
Differently from example 3, the polymer particles were continuously discharged
from the
reactor according to the prior art technique of using a discharge valve,
placed in the bottom
region of polymer fluidized bed. Therefore, during the polymerization run the
control valve
12 and the valve l0c were maintained completely closed, so as to stop the
continuous
recycle of solid in the loop R and also the polymer discharge from the loop R.
The control valve 23 was kept open, so as to discharge polymer via line 22
into the
separation tank 16. The opening of control valve 23 is adjusted so as to keep
constant the
level of solid inside the fluidized bed reactor.
The separator tank 16 was kept at a pressure of 0.5 bar: the pressure
downstream the
discharge valve 23 was of about 2 bar, so as to stop the polymerization
reaction and also to
cause a partial devolatilization of the gas dissolved into the solid
particles.
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Polypropylene granules of spherical shape having an average diameter of about
2.0 mm
were obtained. About 5.7 t/h of polypropylene were continuously discharged via
line 22
into the separation tank 16. The total amount of gas discharged with the
solid, measured at
the top of the separator tank 16, was equal to 420 Nm3/h.
The comparison with Example 3 demonstrates that the process and polymerization
apparatus of the invention is able to lead to a remarkable reduction in
volumes of unreacted
gases (monomer and inert components) to be recovered and recycled to the
reactor.
